Flames in Type Ia Supernova: Deflagration-Detonation Transition in the Oxygen Burning Flame

The flame in a Type Ia supernova is a conglomerate structure which, depending on density, may involve separate regions of carbon, oxygen and silicon burning, all propagating in a self-similar, subsonic front. The separation between these three burning regions increases as the density declines until eventually, below about 2 × 10 g cm, only carbon burning remains active, the other two burning phases having “frozen out” on stellar scales. Between 2 and 3 × 10 g cm however, there remains an energetic oxygen burning region that trails the carbon burning by an amount that is sensitive to the turbulence intensity. As the carbon flame makes a transition to the distributed regime (Karlovitz number >∼10), the characteristic separation between the carbon and oxygen burning regions increases dramatically, from a fraction of a meter to many kilometers. The oxygen-rich mixture between the two flames is created at a nearly constant temperature, and turbulence helps to maintain islands of well-mixed isothermal fuel as the temperature increases. The delayed burning of these regions can be supersonic and could initiate a detonation. Subject headings: supernovae: general; hydrodynamics, shock waves, turbulence